Arene Subunits Research Articles

Solution and Solvent-Free Stopper Exchange Reactions for the Preparation of Pillar[5]arene-Containing [2] and [3]Rotaxanes.

Invited for the cover of this issue are the group of Iwona and Jean-François Nierengarten from the University of Strasbourg (LIMA, UMR 7042, CNRS) and collaborators from the University of Carthage and the IPHC (University of Strasbourg and CNRS, UMR 7178). The image illustrates the fast motions of a pillar[5]arene subunit along the axle of a rotaxane, reminiscent of those of a guitarist's hand along the neck allowing him to use random parts of a scale with certain sweet spots when improvising a solo. Read the full text of the article at 10.1002/chem.202304131.

Solution and Solvent-Free Stopper Exchange Reactions for the Preparation of Pillar[5]arene-containing [2] and [3]Rotaxanes.

Diamine reagents have been used to functionalize a [2]rotaxane building block bearing an activated pentafluorophenyl ester stopper. Upon a first acylation, an intermediate host-guest complex with a terminal amine function is obtained. Dissociation of the intermediate occurs in solution and acylation of the released axle generates a [2]rotaxane with an elongated axle subunit. In contrast, the corresponding [3]rotaxane can be obtained if the reaction conditions are appropriate to stabilize the inclusion complex of the mono-amine intermediate and the pillar[5]arene. This is the case when the stopper exchange is performed under mechanochemical solvent-free conditions. Alternatively, if the newly introduced terminal amide group is large enough to prevent the dissociation, the second acylation provides exclusively a [3]rotaxane. On the other hand, detailed conformational analysis has been also carried out by variable temperature NMR investigations. A complete understanding of the shuttling motions of the pillar[5]arene subunit along the axles of the rotaxanes reported therein has been achieved with the help of density functional theory calculations.

Influence of H-bond competitors on the solvent-dependent structures of an octaurea-calix[4]tube.

A novel octaurea-calix[4]tube (UC4T) has been synthesized in three steps from the original Beer's p-tert-butylcalix[4]tube ionophore. In a polar solvent (DMSO-d6), UC4T rapidly interconverts between two identical conformations with C2v symmetry for the two calix[4]arene subunits. However, in a less polar solvent mixture (CDCl3/DMSO-d6, 98 : 2), UC4T adopts a highly distorted asymmetric structure, which hinders the formation of typical tetraurea calix[4]arene capsular assemblies. The complexation of potassium (or barium) cations inside the dioxyethylene ionophoric binding site of UC4T triggers a C2v to C4v symmetry rearrangement of the two calix[4]arene subunits. This rearrangement leads to the formation of a transient capsular dimeric species observed in solution upon the addition of KI or BaCl2·2H2O to a solution (CDCl3/DMSO-d6, 98 : 2) of the macrocycle. X-ray studies confirm UC4T's ability to adopt different asymmetric conformations, depending on its interactions with solvent molecules. Two distinct crystal forms (α and β) of UC4T have been obtained, each displaying divergent calix[4]arene subunits with pinched-cone conformations. These conformations exhibit distinctive head-to-tail (α) or head-to-head/tail-to-tail (β) orientations of the ureido groups, which are involved in hydrogen bonding with solvent molecules. Notably, the pseudo-capsular 1D supramolecular polymeric arrays observed in the β form of UC4T assemble to create large parallel solvent channels.

Construction of pillar[5]arene-based photochromic supramolecular polymeric system with tunable thermal bleaching rate

Naphthopyran is targeted as the photochromic material here due to its fast photo-sensitivity, high optical density, relative chemical structural simplicity, and its colorless degradation products are also important in commercial ophthalmic applications as variable transmission lenses. In this paper, we reported the synthesis and characterization of a new class of photochromic supramolecular system containing both pillar[5]arene subunit and naphthopyran moiety in the chemical structural backbone. Owing to the supramolecular host-guest interactions of pillar[5]arene/neutral nitrile molecular recognition motif, the designed molecules could be spontaneously self-assembled. Thus, the photochromic supramolecular system indicates an efficient photochromism with convenient tunable fading rate, providing a simple and versatile strategy to fabricate photochromic supramolecular system, as well as potential applications in advanced photo-responsive materials.

The Effects of Structural Modifications of Bis‐tert‐alcohol‐Functionalized Crown‐Calix[4]arenes as Nucleophilic Fluorination Promotors and Relations with Computational Predictions

We prepared four structurally modified bis‐tert‐alcohol functionalized crown ether‐strapped calix[4]arene promoters, that is, bis‐tert‐alcohol functionalized crown‐5‐calix[4]arene (BA5C), bis‐tert‐alcohol functionalized crown‐7‐calix[4]arene (BA7C), bis‐1,1‐dimethylbutanol functionalized crown 6‐calix[4]arene (B3A6C), and bis‐1,1‐dimethylhexanol functionalized crown 6‐calix[4]arene (B5AC6C). Nucleophilic fluorinations were conducted using alkali metal fluorides to investigate phase‐transfer catalytic activities with respect to the sizes of crown ether‐strapped calix[4]arene (CEC) subunits and lengths of alkyl chain spacers between tert‐alcohol and CEC subunits. The reactivity of potassium fluoride was significantly enhanced by BA5C promoter because its crown‐5‐calix[4]arene subunit had cavity size appropriate for K+ capture. In contrast, BA7C did not efficiently enhance cesium fluoride SN2 fluorination because its crown‐7‐calix[4]arene cavity was too large to capture Cs+. The alkyl spacer of bis‐tert‐alcohol functionalized CEC provided sufficient distance between F– and Cs+. In particular, the three‐component alkyl carbon chain of B3A6C was optimal in terms of increasing CsF reactivity. Systematic quantum chemical analysis predicted that fluorination reactions facilitated by bis‐tert‐alcohol‐substituted crown‐6‐calix[4]arene (BACCA) and B3A6C might proceed via separated ion‐pair type pre‐reaction complexes, which suggested that these two promotors act as Lewis bases that essentially separate CsF ion‐pairs. The lower (by 0.4 kcal/mol) Gibbs free energy of activation for the reaction promoted by B3A6C than that facilitated by BACCA concurred with the experimentally observed slightly greater efficiency of B3A6C than BACCA.

Bis-tert-Alcohol-Functionalized Crown-6-Calix[4]arene: An Organic Promoter for Nucleophilic Fluorination.

A bis-tert-alcohol-functionalized crown-6-calix[4]arene (BACCA) was designed and prepared as a multifunctional organic promoter for nucleophilic fluorinations with CsF. By formation of a CsF/BACCA complex, BACCA could release a significantly active and selective fluoride source for SN2 fluorination reactions. The origin of the promoting effects of BACCA was studied by quantum chemical methods. The role of BACCA was revealed to be separation of the metal fluoride to a large distance (>8 Å), thereby producing an essentially "free" F(-). The synergistic actions of the crown-6-calix[4]arene subunit (whose O atoms coordinate the counter-cation Cs(+)) and the terminal tert-alcohol OH groups (forming controlled hydrogen bonds with F(-)) of BACCA led to tremendous efficiency in SN2 fluorination of base-sensitive substrates.

Negative Cooperativity in the Binding of Imidazolium and Viologen Ions to a Pillar[5]arene-Crown Ether Fused Host.

A pillar[5]arene-crown ether fused bicyclic host 1 was found to be able to recognize an imidazolium ion G1 by its pillar[5]arene subunit and a viologen ion G2 by its crown ether receptor discriminatively. The simultaneous binding of G1 and G2 by 1 resulted in the formation of a three-component host-guest complex G1⊂1⊃G2. Negative heterotropic cooperative effects were displayed by G1 and G2 in their binding to 1 and were investigated by stepwise bindings of G1 and G2 to 1.

Langmuir and Langmuir-Blodgett Films from Amphiphilic Pillar[5]arene-Containing [2]Rotaxanes.

Amphiphilic pillar[5]arene-containing [2]rotaxanes have been prepared and fully characterized. In the particular case of the [2]rotaxane incorporating a 1,4-diethoxypillar[5]arene subunit, the structure of the compound was confirmed by X-ray crystal structure analysis. Owing to a good hydrophilic/hydrophobic balance, stable Langmuir films have been obtained for these rotaxanes and the size of the peripheral alkyl chains on the pillar[5]arene subunit has a dramatic influence on the reversibility during compression-decompression cycles. Indeed, when these are small enough, molecular reorganization of the rotaxane by gliding motions are capable of preventing strong π-π interactions between neighboring macrocycles in the thin film.

Ion pair-induced conformational motion in calix[4]arene-strapped calix[4]pyrroles.

In order to understand the still-poorly understood interplay between calix[4]arene conformations and cation and anion recognition in multicomponent systems, the ion pair receptors 1 and 2 were synthesized. In solution and in the solid state, the calix[4]arene subunit of receptor 1 adopts a cone conformation, while that of 2 interconverts between the cone and the partial cone conformation. These geometric features differ from previous systems where the calix[4]arene moiety was locked in the 1,3-alternate conformation. A combination of 1H NMR spectroscopic analyses and single crystal X-ray diffraction studies reveal that receptor 1 binds the fluoride and the chloride anion via significantly different binding modes, displaying, for instance, 1 : 1 and 2 : 3 binding stoichiometries with CsF and CsCl, respectively. In the case of 2, the conformation of the calix[4]arene constituent of 2 is highly dependent on the size and quantity of anions present. For example, upon treatment of 2 with the fluoride anion (as both the TBA+ and Cs+ salts), the calix[4]arene unit coexists as cone and partial cone conformers that are inter-convertible. In the presence of excess CsF, the aromatic rings of the calix[4]arene subunit becomes locked in the pinched cone conformation with the result that an ion pair-mediated coordination polymer is formed. In the presence of excess CsCl, the calix[4]arene unit of 2 adopts only the partial cone conformation stabilized by aryl CH-anion hydrogen bonding interactions. The present systems constitute a rare set of related receptors wherein the effects of conformational changes are so tightly coupled with ion recognition.

Inherently chiral biscalixarene cone–cone conformers consisting of calix[4]arene and calix[5]arene subunits

Inherently chiral biscalixarenes with hetero-cavities were synthesized by a covalent assembly of p-tert-butylcalix[5]arene with a 1,3-substituted calix[4]arene via 1,3-alkylation reaction and subsequent desymmetrization. The racemates were resolved by chiral HPLC method. 1H NMR spectra, VT-NMR spectra, and theoretical calculations support that the calix[5]arene subunit of the inherently chiral calix[4][5]arene ester adopts a cone-in conformation, with the aromatic ring bearing the CH2CO2Et group tilting inward the calix[5]arene cavity. By contrast, such a cone-in structural feature of the calix[5]arene subunit disappears for the corresponding inherently chiral calix[4][5]arene carboxylic acid, due to the intramolecular hydrogen bonding between the carboxyl group and an ethereal oxygen of the glycolic chain.

Theoretical study on the ion–pair recognition of Na+/X− (X = F− , Cl− , Br− ) by urea calix[4]bis crown-3 derivative

The structure of tetraurea calix[4]bis crown-3 derivative (Ht) and its complexes with ion–pair Na+/X− (X = F− , Cl− , Br− ) were studied using quantum mechanical (QM) methods (at the B3LYP/6-31G(d,p) level) and molecular dynamics (MD) simulations under various conditions. Two types of complexes (host-separated type (S) and contact type (C)) were found based on QM calculations in the gas phase. The ion-binding sites in Ht, the interaction forces between Na+/X− and Ht and the recognition selectivity for ion pairs were discussed in detail. In the case of host-separated type (S), the MD simulation results showed that the halogen anion gradually broke away from the host molecule in water. However, the anion got into the calix[4]arene subunit and interacted with sodium cation in chloroform solvent. For the contact type, anion and cation were always embraced by the host molecule during a 1-ns simulation both in water and chloroform. These results indicated that Ht could be a potential selective ion–pair receptor, and the QM+MD strategy could provide reasonable descriptions for ion–pair recognitions.

Self-Assembly of Thiacalix[4]arene-Supported Nickel(II)/Cobalt(II) Complexes Sustained by in Situ Generated 5-Methyltetrazolate Ligand

Solvothermal reactions of thiacalix[4]arene, NaN3, and acetonitrile in the presence of nickel(II)/cobalt(II) salts yielded four discrete complexes sustained by the in situ generated 5-methyltetrazolate ligand, [NiII12(PTC4A)3(μ6-CO3)2(μ-Mtta)2(μ-Mtta)4 (μ4-Mtta)2(Py)4]·7DMF·2Py·dma (1), [CoII12(PTC4A)3(HCOO)3(μ6-CO3)2 (μ-Mtta)(μ-Mtta)2(μ4-Mtta)2(Py)4]·5DMF·dma (2), [CoII12(BTC4A)3(HCOO)2 (μ6-CO3)2(μ-Mtta)4(μ4-Mtta)2(dma)2(Pz)2]·2DMF·3dma (3), and [CoII16(BTC4A)4(μ4-Cl)4 (HCOO)2(μ-Mtta)6(μ-Mtta)8]·10DMF·6CH3CN·4Hdma (4) (H4PTC4A = p-phenylthiacalix[4]arene; H4BTC4A = p-tert-butylthiacalix[4]arene; HMtta = 5-methyl tetrazolate). Crystal structural analyses revealed that complexes 1–3 are stacked by pseudotrigonal planar entities, which consist of three metalII4-thiacalix[4]arene subunits including two shuttlecock-like and one cylinder-like ones. These subunits are connected in an up-to-down-to-up fashion through six different 5-methyl tetrazolate anions. Both the in situ generated 5-methyl tetrazolate anion and carbonato anion play an important role in constructing these high-nuclearity clusters. When the corresponding chloride salt was used as precursors in the synthesis, complex 4 was obtained, which is stacked by wheel-like entities possessing four shuttlecock-like building blocks linked by eight in situ generated 5-methyl tetrazolate ligands in an up-to-up fashion. The differences in the structures of complexes 3 and 4 indicate that the geometry and size of the corresponding anions together with their coordinating properties are essential in determining the final structures. The magnetic properties of complexes 1–4 were examined, indicating strong antiferromagnetic interactions between the nickel(II)/cobalt(II) ions in the temperature range of 50–300 K.

Efficient Synthesis of Maxonine Analogues from N‐Substituted Benzyl‐1‐formyl‐9H‐β‐carbolines

AbstractAcid‐catalyzed Pomeranz–Fritsch‐type reaction between the acetal group at C‐1 and the arene unit of the benzyl group at N‐9 in 9‐substituted benzyl‐1‐(dimethoxymethyl)‐9H‐β‐carboline affords fused‐β‐carbolines that can be readily oxidized to furnish a maxonine‐type framework. Mechanistically, the reaction proceeds via a protonated aldehyde as the intermediate, which undergoes attack by the activated arene subunit of the benzyl moiety. On the other hand, the Morita–Baylis–Hillman adducts of 1‐formyl‐N‐substituted benzyl‐9H‐β‐carbolines undergo an efficient P2O5‐mediated intramolecular Friedel–Crafts reaction between the secondary hydroxy group and the activated phenyl group of the benzyl subunit to yield fused‐β‐carbolines. Investigations into the scope of the substrates reveal that the success of the methodology relies on the degree of activation of the phenyl ring. For substrates having less‐activated phenyl groups, indolizinoindole derivatives were isolated in moderate yields only.

Synthesis and conformational analysis of porphyrin derivatives substituted with calix[4]arene subunits

The synthesis of a complete series of calix[4]arene-substituted porphyrins is described. A detailed conformational analysis based on variable-temperature 1H NMR experiments revealed dynamic phenomena arising from the restricted rotation around the calix[4]arene-porphyrin bond.

Syntheses of Marchantins C, O and P as Promising Highly Bioactive Compounds

Recently, remarkable microtubule inhibitor and anti-tumor activities of the bis(bibenzyl) marchantin C--isolated from liverworts like Marchantia polymorpha since 1983--were found. In this paper we describe the efficient total synthesis of this subtype of bis(bibenzylic) compounds with two biarylether connections. Two selectively methylated derivatives known as natural compounds marchantin O and P were synthesized for the first time by modification of the arene subunits and can now be considered as promising highly bioactive compounds.

Calix[6]arene-Based Cascade Complexes of Organic Ion Triplets Stable in a Protic Solvent

Herein we report a D(3h)-symmetric tail-to-tail bis-calix[6]arene 3 featuring two divergent cavities triply connected by ureido linkages. This calix[6]tube was synthesized by a domino Staudinger/aza-Wittig reaction followed by a macrocyclization reaction. This process also afforded a C(2h)-symmetric isomer that represents a rare example of a self-threaded rotaxane based on calix[6]arene subunits. The binding properties of 3 have been evaluated by NMR studies. Thus, bis-calix[6]arene 3 is able to bind simultaneously two neutral ureido guests through an induced-fit process. The guests are located in the cavities and are recognized through multiple hydrogen-bonding interactions with the ureido bridges. Host 3 can also simultaneously bind multiple ions and is especially efficient for the complexation of organic ion triplets. The anion is recognized through hydrogen-bonding interactions at the ureido binding site and is thus located between the two ammonium ions accommodated in the cavities. The resulting [1+1+2] quaternary complexes represent rare examples of cascade complexes with organic cations. These complexes are unique: 1) They are stable even in a markedly protic solvent, 2) the recognition of the ion triplets proceeds in a cooperative way through an induced-fit process and with a high selectivity, linear cations and doubly charged anions being particularly well recognized, 3) the ions are bound as contact ion triplets thanks to the closeness of the three binding sites, 4) the cationic guests can be exchanged and thus mixed [1+1+1+1] complexes can be obtained, 5) the ureido linkers wrapped around the anion adopt a helical shape and the resulting chirality is sensed by the cations. In other words, bis-calix[6]arene 3 presents a selective inner tunnel in which multiple guests such as organic ion triplets can be aligned in a cooperative way through induced-fit processes.

Crown-6-calix[4]arene-Capped Calix[4]pyrrole: An Ion-Pair Receptor for Solvent-Separated CsF Ions

An ion-pair receptor, 1, containing both cation- and anion-recognizing sites, has been synthesized and characterized. Single-crystal X-ray diffraction structural studies and (1)H NMR spectroscopic analyses confirmed that 1 forms stable 1:1 complexes with CsF in solution and in the solid state in spite of the large separation enforced between the receptor-bound anion and cation. In 9:1 CDCl3/CD3OD, binding of fluoride anion within the calix[4]pyrrole core of 1 was not observed in the absence of a cobound cesium cation; however, it was seen in this solvent mixture under conditions where a Cs(+) cation was bound to the crown ether-strapped calix[4]arene subunit.

On the ‘livingness’ of a dynamic library of cyclophane formaldehyde acetals incorporating calix[4]arene subunits

AbstractThe acid catalyzed transacetalation of cyclophane formaldehyde acetals incorporating calix[4]arene subunits generates a short‐lived dynamic library of macrocycles. The side reaction responsible for the loss of ‘livingness’ is the unexpected decomposition of monomeric units into a bridged ether and formaldehyde. A plausible mechanism is suggested, in which the crucial step is the formation of benzyl carbocations strongly stabilized by the alkoxy substituents at the lower rim of the calix[4]arene moieties. Copyright © 2008 John Wiley & Sons, Ltd.

Syntheses and extraction properties of novel biscalixarene and thiacalix[4]arene hydrazone derivatives

By reacting thiacalix[4]arene with p-tosyloxyethoxylbenzaldehyde 1, 3-bis(benzaldehyde-4-oxyethyloxy)-p-tert-butylthiacalix[4]arene (2) were prepared in yield of 65%. Refluxing compound 2 with aniline, salicylic hydrazide, nicotinic hydrazide and isonicotinic hydrazide, novel ringopening 1,3-bis-arylformyl-hydrazone substituted thiacalix[4]arene derivatives (3a–3d) were obtained in yields of 77–89%. Refluxing compound 2 with o-phenylendiamine, oxalyl dihydrazide, malonic dihydrazide and adipic dihydrazide in “1 + 1” intermolecular condensation mode under diluted condition, novel 1,3-bis-acyl hydrazone-bridged calix[4]arene derivatives (4a–4d) were prepared in good yields. Moreover, by condensating compound 2 with 1,3-bis(hydrazinocarbonyl-methoxy)-p-tert-butylcalix[4]arene (5), the first example of hydrazone-bridged biscalixarene (6) with calix[4]arene and thiacalix[4]arene subunits was facilely synthesized in yield of 90%. The noncompetitive and competitive extracting experiments showed that these novel hosts were good receptors for both metal cations and α-amino acids. Compounds 3a–3d and 4a–4d showed similar binding properties with high extraction percentage but low extracting selectivities. Biscalixarene 6 exhibited not only high extracting abilities but also good extracting selectivities.

Modelling of auxetic networked polymers built from calix[4]arene building blocks

Auxetic materials (i.e. materials with a negative Poisson's ratio) expand laterally when stretched and become thinner when compressed. This unusual yet very useful property arises from the way by which the nano or microstructure of the material deforms when subjected to uniaxial mechanical loads. This paper discusses a novel class of molecular-level auxetic (networked polymers) built from calix[4]arene building blocks. These calix[4]arene subunits are connected in such a way that they mimic the shape of a “folded macrostructure” which is known to exhibit auxetic behaviour. We confirm through force-field based simulations that these newly proposed networked polymers exhibit negative Poisson's ratios, the magnitudes of which can be changed by introducing slight variations in the molecular structure of these polymers. We also develop simple geometry-based models which explain the values of the Poisson's ratios obtained through the force-field based simulations, and which give an insight into the features of the molecular structure that are responsible for the auxetic effect.